Because there is no such thing as a bulletproof vest. There are bullet resistant vests however they are normally only effective against lower velocity rounds commonly found in domestic operations. Military rounds are generally much higher velocity and much larger in caliber. Also, the greater risk to soldiers is generally fragments of other explosives, either grenades, arty rounds or even IEDs.
By in large though soldiers are issued frag vests designed to offer protection from fragments and smaller caliber rounds. These vests also, in most cases, come with removable ceramic or composite plates that are intended to provide maximum protection from military grade small arms. These plates are heavy and are designed to be worn front and back of the chest and are generally effective IF a bullet hits the plate and doesn't strike the soldier from the side.